1. Technical Field
The present invention relates generally to selective epitaxial growth of silicon-germanium (SiGe), and more particularly, to methods and structure for providing a buffer layer for selective SiGe epitaxial growth to provide uniform nucleation.
2. Related Art
Selective silicon germanium (SiGe) epitaxial growth is used for SiGe raised source drain (RSD) or embedded SiGe structures because it allows for improved p-type field effect transistor (pFET) performance due to compressive strain in the channel and lower contact resistance. The compressive strain in the channel enhances the hole mobility.
One challenge relative to SiGe selective epitaxial growth is that it is very sensitive to surface conditions. The higher the germanium (Ge) concentration, the more sensitivity exists. In contrast, selective silicon (Si) epitaxial growth is less sensitive to the surface condition. SiGe selective epitaxial growth on highly doped substrates, e.g., >1×20/cm3, often leads to spotty growth or no growth where the highly doped substrate is exposed to ambient. In this case, a wet chemical clean and a hydrofluoric (HF) acid etch is necessary to remove the native oxide from the surface. Unfortunately, even with these cleanings steps, the highly doped surface reoxidizes easily, which causes a nucleation problem. Surfaces with residue from a spacer reactive ion etch (RIE) also cause spotty growth in selective SiGe epitaxial growth. In one example, where a 300 Angstrom (Å) thick layer of SiGe is desired, only 2 Å are possible for a highly doped (e.g., ˜1×20/cm3) P+ SOI layer, while for an undoped SOI layer, 314 Å of SiGe can be grown.
In view of the foregoing, there is a need in the art for a solution that solves the problems of the related art.